Electron discharge tube circuit



Oct. 22, 1935. K. STEIMEL 2,018,329

ELECTRON DISCHARGE TUBE CIRCUIT Filed NOV. 15, 1933 INVENTOR P4476 [4717466 500866 KARL S f/MEL 7K4 Aim L ATTORNEY Patented Oct. 22, 1935 orrics 2,018,329 ELECTRON DISCHARGE TUBE CIRCUIT Karl Steimel, Berlin, Germany, assignor to Telefunken Gesellschaft fur Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application November 13, 1933, Serial No. 697,782 In Germany November 2, 193,?

4 Claims. 01. 250-27) Recently various types of tubes have been proposed and have been investigated which, by different ways of overcoming space charge obstructions, afford attaining of a very considerable 5 steepness incomparison with the size of the tubes,

such as electron tubes with gas filling for space charge compensation by ions, control cathode tubes for producing a virtual cathode in extreme proximity of the control grid by the aid of two or a greater number of electrodes between cathode and control grid and maintained upon a fixed potential, and electron tubes in breaking field circuits. These tubes in comparison with the equally large present type have a steepness approximately ten times as large. In the present stage of construction of the apparatus their most valuable field of application would be as power tubes of a radio receiver with direct control as regards high frequency. A particular advantage is obtained in that last stage or power tubes directly controlled as regards high frequency absorb a minimum of disturbing influences. However, the above mentioned tubes in their construction as hitherto known are inadequate to accomplish this purpose or they are very ineflicient in attaining this end.

In view of the present stage of tube construction only the anode rectifier (directional amplifier) circuit would be of interest in this case of application. However, the anode rectifier directly controlled for the purpose of a large supply of energy, is very uneconomical since at the low factor of anode current load (which is very small for the small degree of modulation of toclay) and at full utilization of the range of grid control, the anode current range can only to a small extent be utilized as far as audio frequency is concerned. In comparison with an audio frequency control the possibility of utilization in the 40 anode current range is only about V7. The high steepness is only poorly utilized.

The application of grid rectification by which a favorable audio-frequency power utilization is possible, encounters in these tubes difficulties of 45 various degrees. The high steepness of the anode current entails a steep course of the grid current. In these tubes the resistance of the grid-cathode path of the order of only 1000 ohms lies in the grid current region. The grid circuit is subjected to prohibitive damping which can not be uniformly compensated by means of back coupling. In case of gas filled tubes an additional essential difficulty resides in the fact that a negative grid-cathode resistance of the order of 1000 1 10,000 ohms is produced in the region of the gridion-current, so that connection of a circuit, due v to self excitation, would be impossible without special measures. The second reason against the direct grid-rectifier-control of the last stage or power tube is to be found in the fact that an 5 additional anode rectification occurs at the required large amplitudes causing unallowable distortions. In view of the small range of grid control in tubes of great steepness this condition is quickly reached. 10 It is the purpose of the present invention to eliminate the above difficulties in tubes by means of structural measures and to obtain a great economy in the use of tubes of great steepness as last stage power tubes directly controlled as to 1| high frequency.

In accordance with the invention the problem is solved in that the control grid of the tube of great steepness is formed as a double grid, i. e., the control grid consists of two grids insulated 20 from each other and which are entirely identical as regards their control actions. Such double control grids are known as such in grid rectifier tubes and are generally arranged in coplanar relation. The application of the double control 25 grid in power tubes of great steepness according to the present invention, however, solves entirel different technical problems, namely the technically economical, direct control at high frequency of a last stage power tube having the great advantage of minimum sensitivity against disturbances. The double control or coplanar grid tube however is only comparable with an ordinary audion yet having the advantage that it can utilize larger amplitudes at the grid side. In case of gas filled tubes there exists the further essentially advanced feature that the difficulties present, due to the falling grid current characteristic, and forming a main problem of gas filled tubes are avoided to a considerable extent 40 by the use of double grids.

The manner whereby the described difficulties in tubes of great steepness are eliminated will be seen from the following description of the operation: The tube is used as grid rectifier whereby the two control grids operate as regards high frequency in opposite phase. The resistance and condenser combination across which the audio frequency is produced is placed in the connection between the center of the coil of the oscillatory circuit and the cathode. The rectified currents of both grids are added to each other and both grids are controlled in the same sense as regards audio-frequency. Due to the high .56

frequency control of the two grids in opposite phase, the anode current is not influenced at all with respect to high frequency, since the actions of the fields of the two grids are practically entirely compensated in the cathode-anode discharge path. An additional anode rectification with its distortions is thus entirely avoided. The second difiiculty, the grid current damping, is eliminated in view of the following fact. Each coplanar grid has a through grip upon the other which at substantial symmetry, although not having the value of 1, has a value next thereto. By the term through grip is meant the effect or influence of a first electrode through a second i on a third electrode, and is a non-dimensional figure being always less than 1 and indicates the influence of the variation of the potential of the first electrode upon the effective potential of the third electrode, in comparison with the influence of an equally great variation of potential of the second electrode. At a through grip of 1 grid current cannot be produced at all. At a mutual through grip less than 1, and control in phase opposition a current will be developed which is only small in comparison with the control in equal phase. If the through grip has the Value D=1d, in case of control in opposite phase, the grid current per grid has the value Z =s d V wherein s represents the steepness of one grid current in relation to the other grid potential. In fixing d, the grid current and thus also the damping of the circuit can be determined at will. Thus the dynamic grid current characteristic for opposite phase control can be made as flat as may be desired in contrast with the normal characteristic. In this manner it is also possible to essentially increase the negative impedance of the gas filled tube. In using a heavy gas (argon) the'danger of self excitation can probably be eliminated with the necessity of further measures such as a galvanic coupling to the preceding tube.

The dimensioning of the grid bias of the grid resistance and grid condenser is essentially obtainable in accordance with generally known rules. New viewpoints are to be considered only in the case of gas filled tubes. In order to obtain a high audio-frequency sensitivity the working point must be in the region (audio frequency) of the negative resistance, since in the region of the positive resistance even in case of large audio frequency currents only small voltage variations occur. In the negative range of the resistance the grid resistance must be lower than the absolute value of the negative resistance; the highest sensitivity will be obtained if both are adapted to each other as much as possible. In this case a negative grid bias must also be used in order to be able to adjust the working point. a

In the drawing Fig. 1 shows the grid current characteristic of the tube used in applicants system according to the invention; Figs. 2 and 3 represent schematically different forms of the tube that may be employed in the circuit of Fig. 4; and Fig. 4 shows a detector circuit of a receiving set according to the invention.

Referring now to Fig. 1, there is shown the grid current Ig as a. function of the grid voltage Vg in graphic form (heavy line). In order to find the actual grid potential, straight lines (shown light) are drawn in, the slope-of which represents the value of the grid leak resistance Rg, the working point for a given case being determinable from the intersection of said grid leak resistance characteristic and the grid current graph. It can be seen at once that a negative grid voltage is needed in order to get into the drooping range of the grid current where the s sensitiveness is maximum. The actual grid potential is the sum of the grid voltage applied by the input circuit, the constant grid bias and the voltage drop across the grid leak.

' Referring now to Fig. 2 there is an evacuated 10 glass bulb designated as l, which contains a plate 2, two control grids of identical control action 3 and 4 and an indirectly heated cathode, consisting of the heater 5 and the emitting surface 6. The two control grids: 3 and 4 may be 15 arranged e. g. inform of a double thread screw or of a grating of parallel wires alternately connected together. To remove, the space charge obstruction the glass vessel I is filled with a rare gas (e. g. argon) or a metal vapor (e. g. mer- 20 cury) under low pressure.

Another way to compensate the space charge surrounding the cathode 6 is to insert an additional grid 1 between cathode 6 and the control electrodes 3 and 4 as it is shown in Fig. 3. As 25 it is Well known in the art, the space charge grid 1 is to be, connected'to a positive bias respective to the cathode 6.

The Fig. 4 is a skeleton wiring diagram of a detector stage in a receiving set, wherein a tube 30 according to Fig. 3 isused. Corresponding numbers are used for the identical parts. The signals are collected by the antenna l0, which is in ductively coupled to the tuned input circuit ll of the detector stage. The resonant circuit II 35 comprises a variable condenser I 2 and a centrally tapped coil l3, the ends of which are connected to the control grids 3 and 4 respectively. Between the mid point M of the coil l3 and the cathode 6 a grid blocking condenser I 5 is in- 40 serted, which is bridged by a grid leak resistance I6. A grid detection is accomplished as well known from the customary audion circuit. The high frequency voltage is applied in pushpull to both control grids, whereas they are con- 5 trolled in the same sense as regards audio frequency. The output may be led to a low fre quency amplifier by means of a transformer ill or may directly drive a loud speaker.

What I claim is: V 50 1. A circuit arrangement for the detection of high frequency currents comprising a discharge device having a cathode, an anode, a double grid interposed in the space between cathode and v anode and arranged in coplanar relation, and a 55 space charge grid positioned between said coplanar grids and the cathode, a tuned circuit connected to said coplanar grids, a common path for the grid returns comprising a resistance and a shunt condenser connected between the cathode and the tuned circuit, a positive biasing potential connected between the space charge grid and the cathode, and a utilization circuit connected between the anode and the cathode.

2. A circuit arrangement for the detection of 55 high frequency currents according to claim '1, wherein the coplanar grids having through grip upon each other of a value slightly less than unity.

3. A detector-power amplifier stage in a radio 70 receiving circuit comprising an electron discharge tube having a cathode, a pair of control grids and an anode, and means for reducing the space charge between cathode and control grids, said control grids being identical in regard to con- 75 trol, and insulated from each other and having a through grip upon each other of a value slightly less than unity whereby the large grid current damping resulting from the reduced space charge is materially reduced.

4. A detector-power amplifier stage in a radio receiving circuit comprising an electron discharge tube having a cathode and an anode and a pair of control grids interposed in the space between 10 cathode and anode in coplanar relation, a tuned input circuit connected to said coplanar grids, a common path for the grid returns comprising a resistance and a shunt condenser connected between cathode and tuned circuit, an output circuit'connected between cathode and anode, a positively biased electrode interposed between cathode and control grids for increasing the amplification and efiiciency of the circuit, said control grids being so positioned and disposed and having a through grip upon each other of a value slightly less than unity whereby the large grid currents due to the increased amplification is 10 materially reduced.

KARL S'I'EIMEL. 

